1. Field of the Invention
This invention relates to a solid-state element and a slid-state device and, particularly, to a solid-state element and a slid-state device that prevent the separation of electrode and the reduction of bonding strength cause by a thermal stress so as to discharge efficiently light from inside of the device.
2. Description of the Related Art
One of conventional solid-state devices is a light emitting device that an LED (light-emitting diode) element as a solid-state element is mounted on a substrate with a lead frame or a wiring pattern. In the light emitting device using the LED element, it is important that light confined inside the LED element is reduced to enhance the external radiation efficiency in order to have a high-brightness or high-output device.
One of the LED elements is a flip-chip type LED element that a semiconductor layer is formed on a transparent substrate such as sapphire and light is discharged from the side of the transparent substrate. The flip-chip type LED element is excellent in the external radiation efficiency since it does not generate an optical loss in the semiconductor layer or a passivation film. In the device mounting, the flip-chip type LED element is flip-chip bonded to a wiring member such as a lead frame inside a reflow furnace at a temperature of 250 to 300° C.
In recent years, as a solder used for the electrical connection of LED element, a lead (Pb) free solder is researched according to considerations to the environment. The Pb free solder has a melting point higher than a Pb containing solder, and therefore a problem is caused that the emission efficiency lowers due to an increase in thermal stress in the flip-chip bonding of LED element.
Japanese patent application laid-open No. 11-150297 (related art 1) discloses a nitride semiconductor light emitting element that its p-electrode is multilayered to improve the emission efficiency of such a flip-chip type LED element.
The nitride semiconductor light emitting element is composed of a first positive electrode that is in ohmic-contact with a p-GaN based semiconductor layer, and a second positive electrode formed on the first positive electrode. The second positive electrode includes a layer of Au or Pt, which is in contact with the first positive electrode, in order to enable a light-emitting layer located directly under the second positive electrode to emit light ([0012] and FIG. 1 of the related art 1).
However, in the above related art 1, there is a problem that the electrode layer may be separated (peeled) from the semiconductor layer due to a difference in thermal expansion coefficient therebetween under high-temperature conditions such as reflowing, since the first and second positive electrodes are formed in continuous planar face.